A requirement for conventional data transmission is the establishment of well-defined logic thresholds. Metallic wired systems use pre-defined dc logic levels for this purpose. This is unsatisfactory in an optical system where absolute signal levels are not known a priori. The conventional solution is ac-coupling between the receiver and the logic quantizer. With this approach, dc logic threshold levels are established by forming a "signal average" of the received data pulse. Signals above the average are considered as logic ONEs, while signals below the average are logic ZEROs. While ac-coupled receivers work well for continuous data transmission they do not work well for burst mode data transmissions, where the time average of the signal is continuously and unpredictably varying.
By contrast, high-speed, dc-coupled receivers, while ideally suited for burst mode operation, have proven difficult to implement because of the necessity of establishing a logic reference voltage level within a few millivolts of the dc center (one-half of the sum of the minimum and maximum excursions of the data signal) of the received data pulse.
Our prior U.S. Pat. No. 5,025,456 entitled "Burst Mode Digital Data Receiver" and issued Jun. 18, 1991, solved the above problem using a burst mode digital data receiver which adapts to the amplitude of the incoming burst data packet and automatically adjusts the logic threshold voltage to the dc center, ideally during the first bit of the input data burst.
Today, packet data transmissions are now being sent over optical bus communication systems as described in our article entitled "DC-1Gb/s Burst-Mode Compatible Receiver for Optical Bus Applications" by Yusuke Ota, et al., Journal of Lightway Technology, Vol. 10, No. 2, February 1992. In a bus system, the above-described problem is elevated to a higher degree of difficulty, because now the bus medium is time shared by many optical transmitters.
These bus systems impose two new constraints on the prior art receivers. The first is that receivers are likely to receive closely-spaced packet data signals from different transmitters which have widely varying power levels. For example, one packet may arrive with power level of -15 dBm, followed a few bits later by another packet with power level of -35 dBm (one hundred times smaller). The receiver has to be able to handle a wide range of packet amplitudes, separated by only a few nanoseconds in time.
Second, because it is difficult to turn a transmitter's laser optical source "on" and completely "off" very quickly, lasers are typically biased so that they are always slightly on. But when many lasers are mounted on a bus, each of them slightly on all the time, the dc "dark level" light that results can easily be more than some of the weaker ac signals you are trying to detect. So, there is a need for some equivalent of a high-pass filter to get rid of sensitivity to dc light levels. The easiest way to introduce a high-pass filter is simply to ac couple the signal. But that reintroduces all the problems with ac coupling that the burst mode receiver was originally intended to solve.
This invention is directed toward solving the first problem while the above recited related application is directed toward solving the second problem.